The present invention is directed to a new method and apparatus for pre-heating the media employed for filtration of molten metals. It is necessary to pre-heat such media in order to prevent freezing of the molten metal filtrate upon initiation of the filtration process. A considerable amount of sensible heat is required when employing the present day filtration apparatus since most commercially available media have a high heat capacity. It is also particularly important to obtain maximum temperature uniformity during the pre-heating operation since local over-heating of the media significantly degrades the filtration operation and may also damage the media.
The traditional method employed for pre-heating molten metal filter media is by direct use of a gas flame which directly impinges upon the media. In one arrangement, a burner utilizing a mixture of air/natural gas is temporarily and removably placed near the bottom of a fixture employed to contain the reticulated or extruded monolithic ceramic media. The air/gas mixture emitted by the burner is ignited to produce a flame which impinges upon the underside of the media. Pre-heating continues for a prescribed period of time, or until some minimum media surface temperature is reached. At that time, the flame is extinguished, the burner is removed and metal flow through the media is initiated. The flow through the filter media must begin quickly after removal of the burner from the fixture to avoid need for repeating the pre-heat process due to cooling of the media.
Various gas-fired and electrical resistance heated hot air blowers have been used as an alternative to pre-heating the media by direct flame impingement. The expected benefits of the latter approach is to provide greater temperature uniformity and avoid local over-heating of the filter media. However, the limitations of excessive pre-heat time and low attainable temperature severely restricts the use of the forced hot air techniques.
In addition, all of the aforementioned preheat methods require direct access to the filter media and are not amenable to isolated filtration processes such as those which may be employed in a vacuum furnace.